A widely used and successful commercial process for synthesizing acetic acid involves the catalyzed carbonylation of methanol with carbon monoxide. The catalyst contains rhodium and/or iridium and a halogen promoter, typically methyl iodide. The reaction is conducted by continuously bubbling carbon monoxide through a liquid reaction medium in which the catalyst is dissolved. The reaction medium comprises acetic acid, methyl acetate, water, methyl iodide and the catalyst. Conventional commercial processes for the carbonylation of methanol include those described in U.S. Pat. Nos. 3,769,329, 5,001,259, 5,026,908, and 5,144,068, the entireties of which are incorporated herein by reference. Another conventional methanol carbonylation process includes the Cativa™ process, which is discussed in Jones, J. H. (2002), “The Cativa™ Process for the Manufacture of Acetic Acid,” Platinum Metals Review, 44 (3): 94-105, the entirety of which is incorporated herein by reference.
The literature describes the addition of various components to the carbonylation reaction medium in order to increase catalyst stability and/or prevent loss of the catalyst during processing steps. European Patent No. 0 153 834, for example, the entirety of which is incorporated herein by reference, discloses a process for preparing acetic acid by carbonylation of a methanol feed stock in a liquid reaction medium comprising a rhodium catalyst, an iodide or bromide component, an imidazole stabilizing compound, water and acetic acid. Suitable imidazole stabilizing compounds have the general formula:
wherein R1, R2, R3, and R4 are each independently hydrogen, alkyl, aryl, cycloalkyl or alkaryl hydrocarbyl radicals.
U.S. Pat. No. 5,442,107, the entirety of which is incorporated herein by reference, discloses a process for preparing a carboxylic acid by reaction of carbon monoxide with an alcohol in the presence of a rhodium catalyst in a liquid reaction medium comprising: (a) at least a quantity of water of about 0.1 to 15% by weight; (b) a catalyst stabilizer which is a quaternary ammonium iodide of a heterocyclic nitrogen compound selected from the group consisting of 2-ethyl-4-methylimidazole, 4-methylimidazole, 4-tert-butylpyridine, 2-hydroxypyridine, 3-hydroxypyridine and 4-hydroxypyridine; (c) the iodide derivative corresponding to the alcohol, (d) the ester of the carboxylic acid and the alcohol; (e) a rhodium catalyst, and (f) the carboxylic acid.
U.S. Pat. No. 7,115,774, the entirety of which is incorporated herein by reference, discloses a process for carbonylating alcohols with carbon monoxide in the liquid phase, wherein the process comprises a halogenated promoter in at least one non-aqueous ionic liquid comprising at least one salt of the formula Q+A−, in which Q+ represents a quaternary ammonium and/or a quaternary phosphonium cation, and A− is an anion capable of forming a liquid salt below 90° C. Suitable quaternary ammonium and/or quaternary phosphonium cations include N-butylpyridinium hexafluorophosphate, N-ethyl pyridinium tetrafluoroborate, pyridinium fluorosulfonate, 3-butyl-1-methylimidazolium tetrafluoroborate, 3-butyl-1-methylimidazolium hexafluoroantimonate, 3-butyl-1-methylimidazolium hexafluorophosphate, 3-butyl-1-methylimidazolium trifluoromethylsulfonate, 3-butyl-1-methylimidazolium bis(trifluoromethylsulfonyl)amide, trimethylphenylammonium hexafluorophosphate and tetrabutylphosphonium tetrafluoroborate.
U.S. Pat. No. 7,582,792, the entirety of which is incorporated herein by reference, discloses a carbonylation process for the production of a carboxylic acid by combining in a reaction zone a carbonylation feedstock compound comprising an alkanol, a Group VIII metal carbonylation catalyst comprising rhodium (Rh), iridium (Ir), or a compound thereof; an onium salt compound comprising a 1,3-dialkylimidazolium iodide or an N-alkylpyridinium iodide; and carbon monoxide under carbonylation conditions of pressure and temperature, wherein neither hydrogen iodide nor an alkyl iodide exogenous or extraneous to the carbonylation process is added or supplied to the reaction zone.
U.S. Pat. No. 5,144,068, the entirety of which is incorporated herein by reference, discloses a process for producing acetic acid by reacting methanol with carbon monoxide in a liquid reaction medium containing a rhodium (Rh) catalyst and comprising water, acetic acid, methyl iodide, and methyl acetate, wherein catalyst stability is maintained in the reaction by maintaining in said reaction medium during the course of said reaction about 0.1 wt. % to less than 14 wt. % of water together with (a) an effective amount in the range of about 2 wt. % to 20 wt. % of a catalyst stabilizer selected from the group consisting of iodide salts which are soluble in said reaction medium in effective concentration at reaction temperature, (b) about 5 wt. % to 20 wt. % of methyl iodide, and (c) about 0.5 wt. % to 30 wt. % of methyl acetate. Suitable iodide salts may be a quaternary iodide salt or an iodide salt of a member of the group consisting of the metals of Group IA and Group IIA of the Periodic Table.
U.S. Pat. No. 7,005,541, the entirety of which is incorporated herein by reference, discloses a process for producing acetic acid by carbonylation of methanol in a reaction medium having low water content whereby carbon monoxide is reacted with methanol in the presence of a catalyst system comprising a Group VIII metal catalyst, namely rhodium (Rh), iridium (Ir), cobalt (Co), nickel (Ni), ruthenium (Ru), palladium (Pd) or platinum (Pt), a halogen promoter (i.e. an alkyl iodide such as methyl iodide), and a stabilizer/co-promoter. The catalyst system is dissolved in a liquid medium comprising methyl acetate, acetic acid, a finite amount of water (e.g., at least about 0.1 wt. %) and any other compatible solvent. The stabilizer/co-promoter of the catalyst system may be in the form of a soluble salt from an alkali metal, an alkaline earth metal or a quaternary ammonium or phosphonium salt, particularly an iodide or acetate salt, which generates an effective amount of iodide ions in the reaction medium. The amount of iodide present in the reaction medium is between about 2 and 20 wt. %, and the concentration of alkyl iodide (i.e. methyl iodide) in the reaction medium is between about 2 and 30 wt. %. Suitable catalyst stabilizers or co-promoters include lithium iodide, lithium acetate or mixtures thereof.
U.S. Pat. No. 7,678,939, the entirety of which is incorporated herein by reference, discloses a process for producing acetic acid by carbonylation of methanol wherein catalyst stability is achieved by maintaining, in the reaction medium, a rhodium-based catalyst system having a halogen promoter and an iodide salt co-promoter wherein the iodide salt co-promoter has an iodide concentration of greater than about 3 wt. %. Suitable iodide salt co-promoters include a soluble salt of an alkali metal or alkaline earth metal or a quaternary ammonium or phosphonium salt. At least one ruthenium catalyst stabilizer, at least one tin catalyst stabilizer, or mixtures thereof are also incorporated into the catalyst system. The water concentration of the reaction medium is between 0.1 to 14 wt. %.
As reflected above, various processes exist for the production of acetic acid utilizing a catalyst system comprising, for example, a Group VIII metal catalyst, such as a rhodium-based catalyst, in conjunction with a halogen promoter and/or stabilizer. As a promoter and/or stabilizer, the above references disclose the use of a soluble salt from an alkali metal, an alkaline earth metal or a quaternary ammonium or phosphonium salt (e.g., quaternary iodide salts; quaternary ammonium iodides of a heterocyclic nitrogen compound). However, even in view of the above described processes, there still remains a need for novel processes for producing acetic acid with improved catalyst stability.